Conventional vehicle lighting systems typically employ a plurality of lights for various purposes. For example, headlights are used to illuminate the vehicle's path and make the vehicle visible to oncoming traffic at night and during adverse weather conditions. The headlights are selectively energized and deenergized in response to the operation of a main light switch. A dimmer switch, connected in series with this light switch, further allows "high" and "low" beam elements in the headlights to be selectively energized.
A group of lights, collectively known as taillights, are also under the control of the main switch. For example, parking lights are included to make the vehicle visible to traffic behind the vehicle, both when traveling and parked. Brakelights, which are further controlled by actuation of a switch coupled to the vehicle brake pedal, indicate braking of the vehicle. By intermittently energizing the driver side and passenger side taillights in response to, for example, a switch provided on the vehicle steering column, left and right turn signals are conventionally provided to indicate the operator's intent to turn the vehicle.
In addition to the headlights and taillights, most vehicles include a plurality of instrument lights to illuminate the various gauges, meters and indicators on the vehicle dashboard. Compartment lights, including a dome light, glove compartment light, storage compartment light and courtesy lights located at various points throughout the vehicle illuminate the vehicle compartments and typically respond to the operation of switches actuated, for example, by the opening of a car door.
In conventional lighting systems, energy is supplied to the various lights in the system by a DC storage battery. During operation of the vehicle, an alternator coupled to the vehicle engine provides energy to the battery, maintaining the battery in a "charged" condition adequate to ensure the continued operation of, for example, the vehicle lighting and ignition systems. Once the engine is turned off, however, the alternator no longer replenishes energy depleted from the battery by the lighting system. As a result, if the vehicle's lights remain energized for an extended interval of time after the engine is shut off, the battery may no longer be able to operate the engine starter, requiring the vehicle operator to replenish the energy drawn from the battery by, for example, electrically connecting the discharged battery to the battery of a running vehicle. As will be appreciated, this procedure is inconvenient at best.
To prevent the vehicle lighting system from inadvertently depleting the energy stored in the battery in this manner, various circuits have been developed. For example, some automobile electrical systems are equipped with devices that produce an audible alarm when the automobile's headlights remain on after the ignition switch is opened. This alarm prompts the automobile operator to extinguish the headlights or risk the discharge of the battery.
The use of an alarm, however, has several disadvantages. First, if the ambient noise level is high or the operator becomes distracted, it remains possible for the operator to inadvertently allow the battery to become discharged. Second, it may be desirable in some instances to automatically extinguish the headlights a short interval of time after the ignition switch is opened. For example, when the vehicle is driven into a dark parking area or garage, it may be useful to leave the lights on for an interval of time that is sufficient to allow the operator to exit the vehicle and enter an adjoining home.
As an alternative to the use of alarms, various circuits have been developed to automatically extinguish vehicle headlights some interval of time after the vehicle is shut off. For example, (Banker) U.S. Pat. No. 3,963,941 discloses the use of a thermally responsive switch in series with the ignition switch to control the flow of current to a vehicle's headlights. The thermally responsive switch includes a heater element and a two-position switch actuated by a bimetallic element. When the vehicle is started, the thermally responsive switch is in its first, or normal, position and current flows from the battery, through the ignition switch, to both the heater element and the series combination of the thermally responsive switch and headlights. As the temperature of the heater element rises, the bimetallic element eventually switches the thermally responsive switch to its second position. In this position, the flow of current to the headlights is no longer through the ignition switch. Thus, when the ignition switch is opened, current will continue to flow to the headlights. Because the heater element is deenergized, however, the bimetallic element will eventually open, interrupting the headlight circuit.
In (Miller, Sr.) U.S. Pat. No. 3,706,006 a headlight control system is disclosed that includes a solenoid-actuated relay and a thermal relay connected in parallel. The solenoid-actuated relay includes a solenoid and a normally open switch that closes when the solenoid is energized. Similarly, the thermal relay includes a bimetallic switch element that responds to the operation of a heating element. When the vehicle engine is started, current flows through the parallel connection of the thermal relay heater element and the solenoid of the other relay. The current to the headlights passes through the normally open switch closed by the solenoid. As the heating element warms, the bimetallic switch element forms an alternative current path to the headlights. When the engine is shut off, the bimetallic switch element maintains the auxiliary current path until it cools.
(Chaustowich) U.S. Pat. No. 3,514,665 discloses a delay switch circuit that includes a pair of solenoid-actuated, two-position switches and two thermally responsive switches. Upon closure of the vehicle's ignition switch, a first of the solenoid-actuated relays is energized, allowing current to flow through the heater elements of the two thermally responsive switches and through the contacts of the second solenoid-actuated relay to the headlights. As the temperature of a first of the heater elements increases, the associated normally open, thermally responsive switch closes, actuating the second relay. With the contacts of the second relay switched to their second position, energy is then provided to the headlights via the thermally responsive switch rather than the ignition switch. Thus, although the first relay will drop out when the ignition switch is turned off, limiting current to the heater elements, energy will continue to be provided to the headlights. Eventually, the temperature of the first heater element drops sufficiently to allow the contacts of the first thermally responsive switch to open, breaking the headlight circuit.
While these arrangements advantageously employ relatively widely available temperature-actuated switches to effect a delayed extinguishing of the lights, they suffer from several disadvantages. For example, both the Banker and Chaustowich arrangements briefly interrupt the headlight power circuit when the thermally responsive switch is actuated. As will be appreciated, such interruptions can have a deleterious impact on contact life. In addition, the circuit disclosed in the Chaustowich patent is relatively complex. Finally, the Banker and Miller, Sr. arrangements employ the thermally responsive switch, rather than the solenoid-actuated switch, to control the flow of current to the headlights.
In light of these observations, it would be desirable to provide a simple, inexpensive circuit that allows, for example, a vehicle's headlights to be automatically extinguished some interval of time after the vehicle is turned off. The circuit would preferably maintain a continuous flow of current to the headlights until the interval of time expires. Given the failure of most existing vehicle electrical systems to provide such circuits, it would further be desirable to provide a circuit that is easily installable in, or "universally" adaptable to, a wide variety of existing systems.